62 MOTORCYCLE MOJO MAY 2019MOJO GARAGE
tech by Costa Mouzouris
Synchronizing
Carburetors
lthough carburetors have
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cient fuel injection on modern
motorcycles, there are still
plenty of older, carburetted
bikes out there. Carburetors
require some periodic adjustments,
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occasionally on multi-cylinder engines
is carburetor synchronization. On
bikes that have more than one cylinder,
synchronizing the carburetors allows
the cylinders to work in unison as the
throttle is opened, thus allowing all
cylinders to operate with the same
output (some V-twins use only one
carburetor, so this is unnecessary). The
engine therefore operates smoothly and
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out of sync will vibrate, possibly lack
power, and sound, well, off.
The ideal way to synchronize
carburetors is by using a synchronizing
tool, which plugs into the individual
intake manifolds and measures intake
manifold vacuum as the engine runs
and the throttle is operated. The tool
will display the vacuum level for each
cylinder simultaneously, either on a set
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transparent tubes. If the throttles are
not set equally, the vacuum will not
A
Syncing carbs is a must for a smooth
running multi-carb engine
DIFFICULTY LEVEL:
read evenly; the vacuum will be higher
on the cylinder or cylinders on which
the throttle valves are slightly closed,
compared to the cylinders with lower
vacuum readings. The more the throttle
is closed for a given engine speed, the
higher the intake vacuum will be.
Adjusting the throttle valves varies
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tion. If the individual carburetors are far
apart, as on a boxer twin, the adjuster
will be at the throttle cable going to each
carb. Parallel-twins and inline-fours have
individual carburetors lined up side by
side, with screw-type adjusters connecting
the throttle valves between adjoining
carburetors. Some older slide carburetors
have adjusters atop each slide.
If you don’t have a synchronizing
tool, you can still synchronize carbure
tors accurately, though to do so, it’s best
to remove the carburetors from the bike
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or mechanical slides from the rear. It’s
important to note that you must use
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since the vacuum slides don’t move
unless the engine is running.
On a twin, there will be one adjusting
screw; on a four cylinder, there will be
Usually the number two carb is the “base” carb and the other carbs are adjusted after this one is properly set.The liquid in the tubesÀuctuates as the carbs areadjusted giving an accuratevacuum reading.